Inverter size is decided by three ceilings and nothing else: the simultaneous draw of whatever you keep on during an outage, the continuous power your battery can actually hand over, and the limit your property's supply connection places on what may be installed. The size you end up with is the lowest of the three — not the average, and not the biggest one the budget allows. Work them in that order, load list first, and only then open a quote. A comparison that starts with inverter ratings is a comparison of answers to a question nobody has asked yet.
I do this at the kitchen table on a Sunday afternoon, usually with the municipal bill on the left, a notebook in the middle, and a pen I keep losing. The bill is useful up to a point. It tells me what the house consumed over a month. It tells me nothing about what happens inside the same second. A house that draws 900 kWh across July and a house that pulls 6 kW at 18:40 on a winter Tuesday are not the same house, even when they are the same address.
So the bill is where I start, and the list is where I actually work.
The list comes first
I walk the house. Fridge, freezer, fibre router, the few LED lights that stay on, the TV, the garage door motor that runs for eight seconds twice a day. Then the burst loads: kettle, microwave, toaster, iron, hairdryer, the geyser element, the tumble dryer, the pool pump on its timer, the borehole pump that starts whenever a tap opens.
For each one I want two numbers rather than one. The running watts, which is on the label or in the manual. And the starting behaviour. Anything with an induction motor — pool pump, borehole pump, fridge compressor, a workshop extractor — pulls several times its running current for a second or two while it comes up to speed. That inrush is why a list that adds up to a perfectly comfortable figure can still throw an inverter into an overload fault the moment the pump kicks in.
Then the harder part: I mark honestly which of those things I am prepared to lose when the grid drops. That is the backup list. Everything else is a grid load, and the grid can carry it. This single distinction — what stays on versus what is allowed to die — matters more than any other decision in the exercise, because it is the difference between sizing for a house and sizing for a household's routine at 06:20 in winter.
The three ceilings
| Ceiling | What sets the number | How it is quoted |
|---|---|---|
| Backup load draw | The wattage running at the same time, not the sum of everything | kW |
| Battery delivery | Continuous discharge the battery and its management system will sustain | kW |
| Supply connection | Municipal connection size and what the municipality will allow to be registered | kVA, or amps |
Whichever of those three is lowest is the ceiling you are working under. There is no point carrying a number for the other two if the third one is lower.
Ceiling one: the draw that happens at the same time
The number that matters is the simultaneous draw of the backup list, not the total of everything on it. If the list adds to 6 kW but no more than 4 kW is ever running together, then 4 kW is the figure — provided I am being honest with myself about the worst ten minutes of the day rather than the average. In most South African houses the worst case is repeatable: the morning routine, or early evening when the geyser element, the oven and the microwave overlap. Repeatable is what matters. Load shedding does not arrive at a convenient moment.
The other half of this ceiling is inrush, as above. An inverter's ability to survive a start-up surge for a second or two is a separate characteristic from its continuous rating, and it is the one that decides whether the pump can stay on the backup list at all.
Ceiling two: what the battery will hand over, continuously
This is the ceiling people skip, and in my experience it is the one that bites most often. A battery and its management system have a continuous discharge figure, and it is frequently lower than the inverter's rating. A machine that can invert 5 kVA is no use if the battery behind it will only deliver 3 kW all evening. The surplus is not a bonus you get to spend; it is headroom you never actually had.
Two things need separating here. There is the figure the battery can hold for a few seconds, and the figure it can hold for the two-hour evening block. They are different numbers, and only the second one belongs in the calculation. The same applies on the charging side — how fast the battery will accept charge is its own limit, and it governs how quickly the system recovers before the next outage.
Ceiling three: what the connection will allow
The property's supply connection is a physical constraint and a regulatory one. The physical side is the size of the connection and, on a single-phase supply, how much of the house can be moved onto backup before the connection itself becomes the problem. On a three-phase property, backing up one phase while the geyser sits on another often produces a setup nobody expected.
The regulatory side runs through the SSEG application and NRS 097-2-1. Municipalities register an inverter capacity, sometimes expressed relative to the connection, sometimes with an export limit attached. That registration figure is a ceiling in the literal sense: exceed it and the application stalls.
There is one piece of behaviour here that people miss. A hybrid inverter's grid pass-through is separate from its inverter output rating. While the grid is up, the machine passes supply through to the house and charges the battery; it is not inverting the whole load itself. So a modest inverter can sit behind a whole-house load considerably larger than itself, as long as its pass-through rating covers that house. The inverter rating only has to carry the backup list — and the pass-through rating, which is its own number on the datasheet, has to cover everything. When the whole house runs through the machine, pass-through can quietly become the fourth ceiling.
The same machine is quoted in kW and registered in kVA
Before any two suppliers can be compared, the units have to match. Household loads are mostly counted in kW. Inverters are quoted in kVA at a stated power factor. Resistive loads — elements, lights, most electronics — sit close to unity. Motors sit lower, around 0.8 lagging, which is why a house full of pumps and compressors behaves differently from one full of kettles.
The conversion is kW = kVA multiplied by the power factor. At 0.8, a 4 kW load is a 5 kVA demand. So the kW figure on a quote and the kVA figure on the registration form can describe the same box — or two different boxes with a real gap between them. Until both numbers are on the same footing, using the power factor the manufacturer actually states, a supplier comparison is guesswork dressed as arithmetic.
Heat takes a bite out of the brochure number
Rated output is quoted at a reference ambient temperature. A garage in the Karoo in January, or a unit bolted to a west-facing wall in Gauteng, does not live at that temperature from about two in the afternoon onwards. Output derates as the unit runs hot, and manufacturers publish the curves. The honest usable number sits below the one on the box, and it moves with the season.
That is why I leave the last slice of headroom alone rather than sizing to the edge. It is not caution for its own sake. It is what covers a 38 °C afternoon and a battery being asked to work harder because the panels are hot and the roof is dusty.
Where the wrong size announces itself
Undersizing is loud. It shows up as repeated overload faults that cluster at the same time of day, because they are not random — they are a collision between loads that reliably overlap. If the fault log keeps timestamps within the same twenty minutes, the ceiling was calculated with the average instead of the peak.
Oversizing is quiet. It shows up in the purchase price, and in standing losses: the inverter's own idle consumption, plus conversion efficiency that drops away when the machine runs at a small fraction of its rating. A large inverter carrying a fridge and a router through an eight-hour night is not doing the same job it does at half load.
What to do before you compare anything
Build the backup list with both running and starting figures. Apply the three ceilings and take the lowest. Convert everything to one unit using the stated power factor. Check what the registration will allow before you fall in love with a particular capacity. Only then put quotes side by side, normalised to the same units and the same assumptions.
The wiring, the changeover and the Certificate of Compliance sit with a registered contractor, and rightly so. The three ceilings are the part you can work out yourself, at the kitchen table, before anyone arrives to sell you something.
